docs: def loopback_devices():
      '''
      Parse through 'losetup -a' output to determine currently mapped
      loopback devices. Output is expected to look like:
  
          /dev/loop0: [0807]:961814 (/tmp/my.img)
  
      :returns: dict: a dict mapping {loopback_dev: backing_file}
      '''
      loopbacks = {}
      cmd = ['losetup', '-a']
      output = check_output(cmd)
      if six.PY3:
          output = output.decode('utf-8')
      devs = [d.strip().split(' ') for d in output.splitlines() if d != '']
      for dev, _, f in devs:
          loopbacks[dev.replace(':', '')] = re.search(r'\((\S+)\)', f).groups()[0]
      return loopbacks